1. Field of the Invention
The field of the present invention is directed to uncooled thermal microdetectors, and in particular, the present invention relates to thermal detector devices that detect radiation in two or more wavelength bands of the electromagnetic spectrum.
2. Discussion of the Related Art
In the field of infrared (IR) detectors, it is known to provide a two-level bolometer radiation detector having a microbridge on a second plane disposed above a first plane including a silicon substrate. For example, U.S. Pat. No. 5,286,976 (hereinafter "the '976 patent") and U.S. Pat. No. 5,300,915 (hereinafter "the '915 patent") disclose a two-level, infrared bolometer detector device 10 as illustrated in FIG. 1. A bolometer is an electromagnetic radiation detector that operates by absorbing incident electromagnetic energy and converting the absorbed incident electromagnetic energy into heat. The bolometer detector device as disclosed in the '915 and '976 patents has an elevated microbridge detector level 11 and a lower level 12. The lower level 12 includes a semiconductor substrate 13 having fabricated at a top surface 14 components of an integrated circuit (IC) 15, using conventional silicon IC fabrication technology. The components of the IC are coated with a protective layer of silicon nitride 16 which protects the IC from contamination. The elevated microbridge detector level 11 includes a silicon nitride layer 20, a resistive layer 21, a silicon nitride layer 22 disposed over the silicon nitride layer 20 and the resistive layer 21, and an IR absorbing layer 23 disposed over the silicon nitride layer 22. Downwardly extending silicon nitride layers 20' and 22', deposited during the fabrication process, provide sloping support legs for the elevated microbridge detector level 11.
The '915 and '976 patents also disclose that a thin film layer 18 of reflective material can be deposited on the lower level 12, to provide a cavity 26 between the elevated microbridge detector level 11 and the lower level 12. A vertical distance d, between the reflective layer 18 and the upper microbridge detector level 11, is chosen so that incident energy passing through layers 20, 21, 22 and 23 is reflected by layer 18 upwardly and has constructive interference properties with the IR energy initially incident on the upper microbridge detector level 11. In particular the '915 and '976 patents disclose that the distance d is chosen to be substantially a quarter of a wavelength of a wavelength band of operation of the detector device, so that a phase of the reflected energy is coincident with a phase of the incident IR energy on the upper microbridge detector level. The '915 and '976 patents further disclose that the elevated microbridge detector level 11 includes a detector of the incident IR energy wherein the IR absorbing layer 23 and the resistive layer 21 make up an active area (not illustrated) of the detector.
As is known in the art, a sensitivity of the bolometer detector device 10 of FIG. 1 is a function of many factors including an absorption coefficient of each material making up the active area of the device over the desired wavelength band of operation, a physical structure of the detector including the cavity structure 26, a thermal isolation of the active area provided by the microbridge structure, and the like. In FIG. 1, the cavity 26 and the microbridge structure provide isolation of the detector's active area from its surroundings, for example the integrated circuit 15, so as to obtain higher isolation than if the active area were disposed on the top surface 14 of the semiconductor substrate 13. The microbridge structure of FIG. 1 also provides for a larger fill factor than a single level bolometer detector device disposed within the substrate 13, since the detector is disposed on the elevated microbridge detector level and the bus lines, the components of the integrated circuit, and the like are disposed on the lower level 12. It is to be understood that for this specification, the fill factor is defined as a fraction of a pixel area that is the active area of the detector. It is also to be understood that the pixel area is the area containing the bolometer detector device 10, or in other words the area within a plane of the substrate 13 that includes either one or both of the upper microbridge detector level 11 and the IC circuit on the lower level 12. In other words the pixel fill factor is the active area divided by the pixel area. Further it is to be understood that the pixel collecting area of the bolometer detector 10 is the area over which the detector device absorbs energy or in other words the area over which the detector device is responsive to incident energy.
The '915 patent discloses that the active area on the elevated microbridge detector level 11 is maximized in order to maximize the fill factor of the bolometer detector device and to therefore increase the sensitivity of the bolometer detector 10. Thus, the '915 patent discloses increasing sensitivity of the detector device 10 by increasing isolation of the bolometer detector device by placing the active area on the upper microbridge level 11, and by maximizing the size of the active area. In addition, the '976 patent discloses that in order to maximize absorption of incident IR radiation in the operating band, a thickness t of all of the layers 20, 21, 22, 23 and the distance d between the upper level 11 and the reflecting layer 18 are chosen to achieve peak absorption over the desired operating wavelength band. More specifically, the thickness t of the layers 20-23 is chosen to optimize a thermal mass of the microbridge level 11 to achieve peak absorption over the desired operating wavelength band, and the distance d is chosen to achieve constructive interference between any energy not initially absorbed by the active area that is reflected from layer 18, and the IR energy initially incident on the upper microbridge level 11.
One problem with the two-level microbridge bolometer detector 10 of FIG. 1 is that it is constructed to operate with a peak absorption sensitivity over a single operating range of wavelengths. In particular, as discussed above, the peak optical absorption wavelength range is determined by the absorption coefficient of the layers of the active area, by selection of the thickness t of layers 20-23 to have an absorption peak over the desired operating wavelength range, and by the shape of the cavity 26. More specifically the distance d between the upper microbridge detector level and the reflective layer 18 is chosen to provide constructive interference properties. However, there is a need for detectors and detector arrays, for example, in threat warning applications such as armored vehicle defense systems and missile warning systems, which can provide a higher probability of detection and reduced false alarm rates by using the detectors and detector arrays over two separate and distinct wavelength ranges of operation. For example, there is a need to eliminate a problem called contrast inversion which results when objects having different temperatures and emissivities have a same radiant emittance in a spectral band. Accordingly, there is a need to provide a detector that operates over at least two wavelength bands.